1. Field of the Invention
The present invention relates, in general, to a novel copolymer for photoresist and, more particularly, to a copolymer of maleimide derivative and acrylic acid, which is of high etch resistance and thermal resistance and can be used for photoresist in submicrolithography.
2. Description of the Prior Art
Certainly, the recent burst into the high integration of semiconductor devices is greatly dependent on the developmental progress of the techniques for forming fine patterns. For example, photoresist patterns are in general used as masks in etching or ion implantation and their fineness is, therefore, a very important factor in determining the degree of integration.
In this regard, a description will be given of conventional methods for forming fine photoresist patterns, below.
In an example, on a semiconductor substrate whose surface is rugged owing to an infrastructure formed therein is coated a photoresist solution made by dissolving a certain ratio of photoresist agent and resin in a solvent, to give a photoresist film.
Thereafter, a light beam is selectively illuminated upon the photoresist via a light screen pattern formed on a transparent substrate, to selectively polymerize the predetermined part of the photoresist film.
Then, the resulting semiconductor substrate is subjected to soft bake at 80-120.degree. C. for 60-120 sec, followed by the development of the semiconductor substrate. For the development, a weak alkali solution consisting mainly of tetramethylammonium hydroxide (TMAH) is used to selectively remove the exposed/unexposed regions of the photoresist film.
Finally, the semiconductor substrate is washed with deionized water and dried, to obtain a photoresist pattern.
The resolution (R) of the photoresist pattern is proportional to the wavelength (.lambda.) of the light source from a steper and to the process parameter (k) and inversely proportional to the numerical aperture (NA) of the steper, as represented by the following formula: ##EQU1##
Thus, in order to enhance the optical resolution of the steper, a light source with short wavelength may be employed. For example, G-line (436 nm) and i-line (365 nm) stepers have been developed but show limited process resolutions of about 0.7 and 0.5 .mu.m, respectively.
In another example, rather than a mono-layer resist, a trilayer layer resist (hereinafter referred to as "TLR") which comprises two photoresist layers and an intermediate layer therebetween is used. Because the process parameter is small, this TLR process shows a more enhanced resolution by 30% than that obtainable in the mono-layer process, allowing the formation of the fine patterns 0.25 .mu.m in size. But, the patterns as small as about 0.2 .mu.m, necessary for the highly integrated semiconductor devices with a scale of 256 M or 1 G DRAM, are beyond this TLR process's ability.
In an effort to break such deadlock, a silylation process in which silicon is selectively injected onto photoresist was developed to lower the limit value of the resolution. This process is, however, complicated and poor in reproductivity.
For the formation of finer patterns smaller than 0.5 .mu.m, a contrast enhancement layer (CEL) in which an additional thin film is formed on a wafer to enhance image contrast or a phase shift mask is used. However, the CEL process is found to be poor in production yield in addition to being complicated.
In the last decade, intensified research has been directed towards finding new light sources suitable to improve the resolution power. As a result, deep ultraviolet (DUV) rays were developed as light sources for the integration of semiconductor devices to 1 giga or higher scales. Now, the exposure process is prevailingly carried out in a steper using as a light source a KrF laser with a wavelength of 248 nm or an ArF laser with a wavelength of 193 nm. In fact, DUV is emerging as the simplest and strongest means, although there is an apparent limit in the recruitment of short wavelengths as a light source. In accordance with the development of the steper utilizing DUV, photoresist for DUV should be developed as well.
Typically, a photoresist for DUV is composed of aromatic polymer and photoacid generator, as represented by the following formula: ##STR2##
As seen in the formula, when the photoresist is exposed through a mask to uv light, the photoacid generator generates an acid (H.sup.+) which subsequently reacts with resin I. As a result, resin I is decomposed into resin II, producing another acid (H.sup.+) which can react, in chain, with another resin I.
While resin II is dissolved out by a developing solution, resin I, unaffected by acid, remains as it is. In this mechanism, a positive image patterned after the mask is formed on the substrate.
KrF, a light source in current use, has a wavelength of 248 nm but does not satisfy the needs for 4G DRAM. For the fine patterns of the much higher densities suitable for 4G DRAM, shorter wavelengths should be employed, and ArF light source 193 nm in wavelength is developed. In practice, however, there has not yet been developed a photoresist which can be applied for the lithography using the ArF light source. The reason is that aromatic resins, showing good etch resistance, are incompatible with the ArF light since aromatic resins absorb the light of 193 nm. The absorption of the light reduces the useful light which the photoacid generator can utilize.
As a result, most of the research concerning ArF photoresist are directed towards polymethylmetacrylate (PMMA) resins, which are represented by the following structural formula: ##STR3##
In contrast with aromatic resins, PMMA resins are poor in etch resistance and thermal resistance and thus, difficult to use in practice.